1. Field
The present application relates generally to laser systems and, more specifically, to calibration of a photoelectromagnetic sensor in a laser source of a laser produced plasma (LPP) extreme ultraviolet (EUV) system.
2. Related Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 102 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to generate EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be generated by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site within an LPP EUV source plasma chamber.
FIG. 1 illustrates some of the components of a prior art LPP EUV system 100. A laser source 101, such as a CO2 laser, produces a laser beam 102 that passes through a beam delivery system 103 and through focusing optics 104 (comprising a lens and a steering mirror). Focusing optics 104 have a primary focus point 105 at an irradiation site within an LPP EUV source plasma chamber 110. A droplet generator 106 produces droplets 107 of an appropriate target material that, when hit by laser beam 102 at the primary focus point 105, generate a plasma which irradiates EUV light. An elliptical mirror (“collector”) 108 focuses the EUV light from the plasma at a focal spot 109 (also known as an intermediate focus position) for delivering the generated EUV light to, e.g., a lithography scanner system (not shown). Focal spot 109 will typically be within a scanner (not shown) containing wafers that are to be exposed to the EUV light. In some embodiments, there may be multiple laser sources 101, with beams that all converge on focusing optics 104. One type of LPP EUV light source may use a CO2 laser and a zinc selenide (ZnSe) lens with an anti-reflective coating and a clear aperture of about 6 to 8 inches.
The laser source 101 can be operated in a burst mode where a number of light pulses are generated in a burst with some amount of time between bursts. The laser source 101 may comprise a number of lasers that generate pulsed laser beams having distinct properties, such as wavelength, and/or pulse length. Within the laser source 101, the beam delivery system 103, and the focusing optics 104, the separate laser beams may be combined, split, or otherwise manipulated.
Before the laser beam 102 reaches the LPP EUV source plasma chamber 110, the beam 102 is measured at various points within the laser source 101, the beam delivery system 103, and/or the focusing optics 104. The measurements are taken using a variety of instruments that measure one or more aspects of the laser beam 102. In some instances, the laser beam 102 may be measured before it is combined with other generated beams or after it has been combined. The instruments, however, may not directly measure certain properties of the laser beam 102 or may be not be calibrated in such a way as to measure the properties of the laser beam 102.